A method and image-forming apparatus for forming two or more toner images on a rotating charge retentive medium can be used, for example, to apply to a print not only a black toner but a toner having an accentuating colour. In the case of more than two developing means it can also be used to apply more colors in order to produce a full-colour print. Since instead of being collected on an extra collecting medium or on the print material, the various toner images are now collected in register on a charge retentive medium itself, so that a compact and relatively inexpensive apparatus is obtained. Furthermore, better register is possible since there is no transfer of the separate toner images to a collecting intermediate medium of this kind.
The problem in collecting a plurality of toner images on a charge retentive medium, however, is that the toner applied by the first developing means must pass the second developing means. The first toner layer applied may be disturbed by the second developing means. To obviate this as far as possible, special additional steps are necessary. One of the steps is usually contactless "scavengeless" development of toner by disposing a wire having an AC voltage applied thereto, for example next to a developing roller. This causes a toner cloud. The disadvantage of this is that it is complicated to perform in the case of wide formats.
U.S. Pat. No. 4,847,655 (the '655 patent), for example, describes a tri-level xerographic developing system for developing a plurality of toner images on a charge retentive medium constructed as a photoconductor. The photoconductor in this case is first charged uniformly, preferably negatively, by a corona, whereafter three separate charge levels are obtained on the photoconductor by tri-level laser exposure. The non-discharged areas, corresponding to a maximum negative charge, are intended for development of positively charged black toner (Charged Area Development) while the maximum discharged areas are intended for development of negatively charged colour toner (Discharged Area Development). The half-discharged areas are not intended for toner development.
Black and colour toner respectively are developed in a first developing station and a second developing station situated downstream, respectively, by a pair of magnetic rollers. In this case the toner is charged triboelectrically by suitable carrier particles, i.e., the toner is a binary toner having two components, namely toner particles and carrier particles. It is stated in the '655 patent that development with an insulating magnetic brush has really not been found suitable for tri-level development since charge fields occur at the edges of a first developed image so that toner intended for a second image is developed here. On the other hand, it is stated in the '655 patent that thin lines are however developed less satisfactorily with a (more) conductive magnetic brush. In the embodiment described, therefore, by the use of adapted toner concentrations, charge levels of the toner and developing distances, development is carried out as well as possible with relatively more conductive magnetic brushes. The electrical conductivity of the toner as measured in a Gutman conductivity cell is in the range from 1.times.10.sup.-9 to 1.times.10.sup.-13 (ohm.cm).sup.-1.
In order to limit the disturbance of the first developed image by the second developed image, the two magnetic rollers in the second developing station have magnetic fields which are specially designed to differ from one another.
The special construction of the second developing station, however, increases costs, while the use of toner which is relatively somewhat less insulating necessitates extra and hence limiting development settings.
U.S. Pat. No. 5,061,969 (the '969 patent) describes another embodiment of a tri-level xerographic developing system with a charge retentive medium again in the form of a photoconductor. In this case, instead of two developing stations both having an insulating magnetic brush or, as in the embodiment in the above-described U.S. Pat. No. 4,847,655 (two relatively less insulating magnetic brushes), a combination of an insulating magnetic brush and a relatively less insulating magnetic brush is used. In this case, the term less insulating denotes a conductivity of less than 1.times.10.sup.-13 (ohm.cm).sup.-1 and insulating denotes a conductivity between 1.times.10.sup.-13 and 1.times.10.sup.-15 (ohm.cm).sup.-1. The developer (toner and carrier) is again of the binary type. In this case colour is first developed with the less insulating magnetic brush and then black with the insulating brush. In the developed colour image, there will be less large electrical edge fields than is the case when developing with an insulating magnetic brush. This reduces the risk of unwanted development of black at colour edges. On the other hand, thin lines and sharp edges can be obtained in the case of black, by development thereof with an insulating magnetic brush. The '969 patent also describes an optional charge device in the form of a scorotron corona situated between the two developing stations. This serves to bring the developed colour image to the same potential as the background level for white. Unwanted electrical edge fields are also reduced further here.
The disturbance of the first developed colour image by the second developing station is admittedly reduced, but it is still present, as is evident from the described extra step of the charge device between the two developing stations.
U.S. Pat. No. 5,367,327 (the '327 patent) describes, for example, a tandem device of a tri-level xerographic developing system with a quad-level xerographic developing system. With the quad-level developing system, four charge levels are produced on a photoconductor, so that blue, yellow and black toner can be developed. With the tri-level system, magenta and cyan toner are then developed. Since the developing station for tri-level development (which is arranged as the second developing station) uses an exposure in the red or infrared wavelength range, the photoconductor may also be exposed at those places where yellow toner is present that has already been developed by the quad-level developing station (which is arranged as the first station). The yellow toner is the only one of the toners that is transparent to this wavelength. By depositing magenta toner on the insulating yellow toner with the tri-level developing station, red is obtained, and by depositing cyan toner on the yellow toner green is obtained. In this way a full-colour print can be obtained.
For full-colour, therefore, as regards wavelength, a different exposure must be chosen for a developing station compared with laser exposure.